thunderbolt: debugfs: Implement Gen 4 margining eye selection

Add a debugfs knob for USB4 Gen 4 margining eye selection. Gen 4 uses
3-level pulse amplitude modulation (PAM3) which changes how margining
measurements are made because PAM3 has two eyes per lane from which
the margins can be measured.

Signed-off-by: Aapo Vienamo <aapo.vienamo@iki.fi>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>